Majoranas Arrive When a negatively charged electron meets a positron—its positively charged antiparticle—they annihilate each other in a flash of gamma rays. A Majorana fermion, on the other hand, is a neutral particle, which is its own antiparticle. No sightings of a Majorana have been reported in the elementary particle world, but recently they have been proposed to exist in solid-state systems and suggested to be of interest as a quantum computing platform. Mourik et al. (p. 1003 , published online 12 April; see the cover; see the Perspective by Brouwer ) set up a semiconductor nanowire contacted on each end by a normal and a superconducting electrode that revealed evidence of Majorana fermions.

Double quantum dot in the few-electron regime is achieved using local gating in an InSb nanowire. The spectrum of two-electron eigenstates is investigated using electric dipole spin resonance. Singlettriplet level repulsion caused by spin-orbit interaction is observed. The size and the anisotropy of singlet-triplet repulsion are used to determine the magnitude and the orientation of the spin-orbit effective field in an InSb nanowire double dot. The obtained results are confirmed using spin blockade leakage current anisotropy and transport spectroscopy of individual quantum dots.PACS numbers: 73.63. Kv, 85.35.Be The spin-orbit interaction (SOI) describes coupling between the motion of an electron and its spin. In one dimension, where electrons can move only to the left or to the right, the SOI couples this left or right motion to either spin-up or spin-down. An extreme situation occurs in what is called a helical liquid [1] where, in the presence of magnetic field, all spin-up electrons move to the left and all spin-down electrons to the right. As proposed recently [2,3], a helical liquid in proximity to a superconductor can generate Majorana fermions [4]. The search for Majorana fermions in 1D conductors is focused on finding the best material in terms of a strong spin-orbit interaction and large Landé g-factors. The latter is required for a helical liquid to exist at magnetic fields that do not suppress superconductivity. High g-factors of the order 50, strong SOI and the ability to induce superconductivity put forward InSb nanowires [5,6] as a natural platform for the realization of 1D topological states.The SOI can be expressed as an effective magnetic field B SO that depends on the electron momentum. An electron moving through the wire undergoes spin precession around B SO with a π rotation over a distance l SO called the spin-orbit length (see Fig. 1(a)). The length l SO is a direct measure of the SOI strength: a stronger SOI results in a shorter l SO . In this letter, we use spin spectra of single electrons in quantum dots [7] to extract l SO and the direction of B SO . In quantum dots, the SOI hybridizes states with different spin [5,8,9]. For a single electron, the SOI-hybridized spin-up and spin-down states form a spin-orbit qubit [10,11]. For two electrons SOI hybridization induces level repulsion between singlet and triplet states. The resulting level-repulsion gap between the well-defined qubit states can be used to measure the SOI: the gap size is determined by l SO [5,8,9] and the gap anisotropy indicates the direction of B SO [12][13][14]. Double quantum dots in InSb nanowires are defined by local gating (Figs. 1(b),1(c)). A finite voltage is applied across the source and drain electrodes; and the current through the nanowire is measured. Five gates underneath the wire create the confinement potential and control the electron number on the two dots [9,15]. We focus on the (1,1) charge configuration ( Fig. 1(d)), in which both the left and the right dot contain exactly one electron, each of them...

Semiconductor nanowires provide an ideal platform for various low-dimensional quantum devices. In particular, topological phases of matter hosting non-Abelian quasiparticles can emerge when a semiconductor nanowire with strong spin-orbit coupling is brought in contact with a superconductor 1,2 . To fully exploit the potential of non-Abelian anyons for topological quantum computing, they need to be exchanged in a wellcontrolled braiding operation 3-8 . Essential hardware for braiding is a network of singlecrystalline nanowires coupled to superconducting islands. Here, we demonstrate a technique for generic bottom-up synthesis of complex quantum devices with a special focus on nanowire networks having a predefined number of superconducting islands.Structural analysis confirms the high crystalline quality of the nanowire junctions, as well as an epitaxial superconductor-semiconductor interface. Quantum transport measurements of nanowire "hashtags" reveal Aharonov-Bohm and weak-antilocalization effects, indicating a phase coherent system with strong spin-orbit coupling. In addition, a 2 proximity-induced hard superconducting gap is demonstrated in these hybrid superconductor-semiconductor nanowires, highlighting the successful materials development necessary for a first braiding experiment. Our approach opens new avenues for the realization of epitaxial 3-dimensional quantum device architectures.Majorana Zero Modes (MZMs) are predicted to emerge once a superconductor (SC) is coupled to a semiconductor nanowire (NW) with a strong spin-orbit interaction (SOI) in an external magnetic field 1,2 . InSb NWs are a prime choice for this application due to the large Landé g-factor (~50) and strong Rashba SOI 9 , crucial for realization of MZMs. In addition, InSb nanowires generally show high mobility and ballistic transport [10][11][12] . Indeed, signatures of Majorana zero modes (MZMs) have been detected in hybrid superconductor-semiconductor InSb and InAs NW systems 11,[13][14][15] . Multiple schemes for topological quantum computing based on braiding of MZMs have been reported, all employing hybrid NW networks 3-8 .Top-down fabrication of InSb NW networks is an attractive route towards scalability 16 , however, the large lattice mismatch between InSb and insulating growth substrates limits the crystal quality. An alternative approach is bottom-up synthesis of out-of-plane NW networks which, due to their large surface-to-volume ratio, effectively relieve strain on their sidewalls, enabling the growth of single-crystalline NWs on highly lattice-mismatched substrates [17][18][19] .Recently, different schemes have been reported for merging NWs into networks [20][21][22] .Unfortunately, these structures are either not single-crystalline, due to a mismatch of the crystal structure of the wires with that of the substrate (i.e. hexagonal NWs on a cubic substrate) 22 , or the yield is low due to the limited control over the multiple accessible growth directions (the yield decreases with the number of junctions in the network) 23 ....

Semiconductor nanowires have opened new research avenues in quantum transport owing to their confined geometry and electrostatic tunability. They have offered an exceptional testbed for superconductivity, leading to the realization of hybrid systems combining the macroscopic quantum properties of superconductors with the possibility to control charges down to a single electron. These advances brought semiconductor nanowires to the forefront of efforts to realize topological superconductivity and Majorana modes. A prime challenge to benefit from the topological properties of Majoranas is to reduce the disorder in hybrid nanowire devices. Here we show ballistic superconductivity in InSb semiconductor nanowires. Our structural and chemical analyses demonstrate a high-quality interface between the nanowire and a NbTiN superconductor that enables ballistic transport. This is manifested by a quantized conductance for normal carriers, a strongly enhanced conductance for Andreev-reflecting carriers, and an induced hard gap with a significantly reduced density of states. These results pave the way for disorder-free Majorana devices.